1. Field of the Invention
This invention relates to a reflector for a lighting device and an illumination system for use in a projection apparatus. In particular, the invention relates to a reflector that can effectively eliminate a centrally dimmed area generated from the opening of the lighting device in an illumination system of a projection apparatus.
2. Descriptions of the Related Art
The projection apparatus, which is widely used in offices, homes, conference rooms or the like, typically comprises a light source and other optical components such as prisms and imaging lens. Because uniform light rays are pertinent for the imaging quality of the projection apparatus, the light source is one of the critical components in the projection apparatus.
Among various types of projection apparatuses, the digital light processing (DLP) projection apparatus, which can generate a sequence of digital light pulses in response to a digital signal, has been popular on the market. An internal arrangement of a conventional DLP projection apparatus is depicted in FIG. 1A. The projection apparatus 10 comprises a light source 11, a light integration rod 12, a color wheel 13, a light-coupling device 14, a digital micro-mirror device (DMD) 15, and a lens 16. Here, only a single DMD 15 and a single light source 11 are shown as an example. The ultra high-pressure (UHP) lamp, which costs less and has a high light emitting efficiency, is the preferred light source 11.
In the configuration depicted in FIG. 1A, the color wheel 13 is adapted to convert light emitted from the light source 11 into light of the three primary colors. Specifically, when the light passes through the light integration rod 12, it is preliminarily turned into images. Then, the color wheel 13, typically disposed at the entrance of the light integration rod 12, generates the three primary colors selectively. Subsequently, upon entering the light coupling device 14, the light is reflected and modulated by individual micro-mirror structures of the DMD 15 and is finally projected by the projection lens 16.
To improve the luminance of the illumination system, a multi-light-source arrangement may be considered so that light from multiple light sources are projected into the light integration rod 12 together for light integration before being used in other back-end optical elements. However, due to the restriction imposed by the étendue conservation law in the DMD 15, a simply addition of light sources will not result in an equivalent improvement of the luminance as desired. Consequently, the configuration of the light sources to achieve an optimal effect has become one of the greatest challenges posed to the designers of illumination systems.
In consideration of the restriction imposed by the étendue conservation law, a conventional way for improvement is to design the optical path as a retro-reflect system. As an example, two parabolic lighting devices 11a, 11b that can emit parallel light are illustrated in FIG. 1B. With the two parabolic lighting devices 11a, 11b disposed opposite of each other, a portion of the parallel light projected from the parabolic lighting devices 11a, 11b is outwardly reflected by a reflecting prism 17 directly, while the remaining portion is first reflected inside the opposite lighting device and then guided to reflect from the reflecting prism 17. As a result, a portion of the light may be reclaimed before being reflected, thus meeting the restriction imposed by the étendue conservation law on the system.
However, some drawbacks still exist with the retro-reflect system. For example, if a lighting device with a bulb (e.g. an UHP lamp) is employed, limitations of the structure may lead to a centrally dimmed area in the light projected from the opening of the lighting device. For example, if an elliptical lighting device 11c is used, as shown in FIG. 1C, a dim area will occur in the center 115 of the projection plane 113 formed by the light projected from the opening since the lighting device 11c comprises a bulb 111. The dimmed area decreases the overall luminance.
In view of this, it is highly desirable to provide a reflector that can effectively eliminate the occurrence of a centrally dimmed area in an illumination system of a projection apparatus.